Topic: RNN (Recurrent Neural Networks) – Part 1 of 4: Introduction and Core Concepts

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1. What is an RNN?

• A Recurrent Neural Network (RNN) is a type of neural network designed to process sequential data, such as time series, text, or speech.

• Unlike feedforward networks, RNNs maintain a memory of previous inputs using hidden states, which makes them powerful for tasks with temporal dependencies.

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2. How RNNs Work

• RNNs process one element of the sequence at a time while maintaining an internal hidden state.

• The hidden state is updated at each time step and used along with the current input to predict the next output.

$$
h_t = \tanh(W_h h_{t-1} + W_x x_t + b)
$$

Where:

• $x_t$ = input at time step t
• $h_t$ = hidden state at time t
• $W_h, W_x$ = weight matrices
• $b$ = bias

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3. Applications of RNNs

• Text classification
• Language modeling
• Sentiment analysis
• Time-series prediction
• Speech recognition
• Machine translation

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4. Basic RNN Architecture

• Input layer: Sequence of data (e.g., words or time points)

• Recurrent layer: Applies the same weights across all time steps

• Output layer: Generates prediction (either per time step or overall)

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5. Simple RNN Example in PyTorch

import torch
import torch.nn as nn

class BasicRNN(nn.Module):
    def __init__(self, input_size, hidden_size, output_size):
        super(BasicRNN, self).__init__()
        self.rnn = nn.RNN(input_size, hidden_size, batch_first=True)
        self.fc = nn.Linear(hidden_size, output_size)

    def forward(self, x):
        out, _ = self.rnn(x)  # out: [batch, seq_len, hidden]
        out = self.fc(out[:, -1, :])  # Take the output from last time step
        return out

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6. Summary

• RNNs are effective for sequential data due to their internal memory.

• Unlike CNNs or FFNs, RNNs take time dependency into account.

• PyTorch offers built-in RNN modules for easy implementation.

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Exercise

• Build an RNN to predict the next character in a short string of text (e.g., “hello”).

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#RNN #DeepLearning #SequentialData #TimeSeries #NLP

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